I rebuilt those carbs back before I retired. That vacuum pull-off feature only goes a little ways.
Try this! It'll only cost you a few minutes time. Get the engine started and warm it up until the chokes are full off and adjust your static mixture and hot idle speed(Now is a good time to find the proper timing and base idle throttle settings. Lean out the air/fuel mixture settings if needed to get a good smooth idle and correct acceleration characteristics). Choose which carb you want to be the "slave" carb. Remove the screw that holds the fast idle cam and link and set that assembly aside. Set both carbs so that warm throttle idle position is the same on both. Set the fast idle on the "master" carb. (This will only work "WELL" if your throttle link between the carbs is tight. NO SLACK OR SLOP!! Small "heim" joints and threaded rod is best!) Now set your choke stove assemblies with the throttle opened half way, engine off and cold, so that they barely close, but close all the way. Open the throttle about one quarter and then open the choke on the master carb a little and see if both throttles close slightly farther when released. If they do, you're headed in the right direction.
Now you're ready to try it out. If you do this, let me know how it works. Like I said, I only ever ran three of these at the time and we removed the entire choke assemblies on the front and rear units, blades, rods, stoves, and linkages! I had to plug the choke pull-off vacuum circuits too.
